The entire Real Life section had to go—not because Real Life examples should be disallowed, but because none of them really fit, most being meta-examples where the question was only asked by tropers in retrospect:
Or removing themselves from the gene pool through... other methods.
Hannibal's march over the Alps into Italy. The Romans were caught completely flat-footed by it because they didn't think anyone would be stupid enough to try.
In fairness, even he lost a not insignificant amount of his army in doing so, so the Romans were somewhat justified in their belief.
Likewise the Nazi conquest of France in 1940. The French left hardly any troops covering the Ardennes sector... because no one would be stupid enough to try and launch an attack through such rough terrain. As with Hannibal, it's not that they were stupid enough to try something impossible; it's that they were good enough to make it possible.
The fact that the Allies left almost no troops there when it was recaptured in 1944 allowing the Nazi's to do the same thing again does makes the Ardennes a case of the Allies being Too Dumb to Live as well.
Charles XII of Sweden's invasion of Russia. Napoleon's invasion of Russia. And later, Hitler's invasion of Russia. Hint: do not start a land war in Russia.
Especially when it's winter in Russia (as was the case with Hitler).
Actually, he started the invasion in June. It took longer than expected. The Nazis actually expected to launch their attack on May 1, but had to delay for six weeks to conquer Yugoslavia. Many historians speculate that the extra six weeks of warm weather in Russia might have resulted in victory.
Only one man has actually successfully conquered Russia. That was Batu Khan (who was almost as badass as his grandfather Genghis). And Russia wasn't united at the time.
Though by not being dumb enough to try in the middle of another war anything more than a short campaign to punish princes who killed his ambassadors. Being strong and unobtrusive made Horde attractive, so when the prince of the richest Russian city had to choose whether to deal with them or the Crusaders, he bothered to travel all the way to Batu and became his son's "blood brothers" to join as a client-state.
Somehow, people are still falling for the Nigerian Scam.
Some 419-scammers fall for the craziest stuff scambaiters tell them.
Ditto for diploma mills — phony universities that thrive on the USA's legal voids and loopholes, which sell diplomas without even teaching anything. Google about "diploma mill" and you'll see many stories of people who actually believed they were now experts in computer science after buying their diplomas without even studying anything, who were shocked beyond belief when they found out their diploma was rendered null and void after the "university" was raided.
Who would be stupid enough to launch an invasion of Finland during a harsh winter, while deliberately not using any soldiers from similarly cold regions near the Finnish border (out of paranoid fears that they'd be too sympathetic with the Finns)? Josef Stalin, that's who. The result was technically a Soviet victory, but only through overwhelming the Finnish defenses through sheer numbers. As one Soviet general put it, "We have won enough ground to bury our dead." This embarrassing performance against the vastly smaller and more poorly equipped Finnish Army, incidentally, was a major factor in Adolf Hitler thinking he could duplicate Batu Khan's feat of conquering Russia. (He couldn't.)
Stalin himself invoked this trope almost word for word when aviation minister Mikhail Khrunichev and aircraft designer A. S. Yakovlev proposed to buy advanced jet engines from the British. "What fool will sell us his secrets?" At the time jet engines were revolutionary technology and selling them would be the equivalent of selling stealth technology to your worst enemy today. Turned out the British Labour government was that stupid. They sold the Rolls Royce Nene to the Soviets who put it in the MIG 15. The Mig gave air superiority to the North Koreans during the first stages of The Korean War, before the appearance of the F-86 Sabre. Just to add insult to injury, they never paid the British the licensing fees either.
Operation Grief: Otto Skorzeny's infamous reputation as a commando was so well known and feared that even though he and his unit had failed to complete their main objective, the fact that they were behind allied lines wearing captured U.S. and British Army Uniforms caused the americans to not only bring about a heavy security crackdown but also to believe anything that his soldiers said when they captured them. They even made up a rumor that Eisenhower and his staff were in danger of being captured. A bold-faced lie that ruined the General's 1944 Christmas because they had completely bought it.
The entire Real Life section had to go—not because Real Life examples should be disallowed, but because none of them really fit, most being meta-examples where the question was only asked by tropers in retrospect: